A bit about myself - I'm a female student from HK studying in the US. I mainly listen to classical, instrumental and pop, and in general I tend to enjoy headphones that are balanced with a touch of warmth. I also value a wide soundstage, accurate positioning, and to a slightly lesser degree clarity and micro-detail retrieval. I should also mention that I'm not very tolerant towards sibilance and shrill treble in general.
Alternate link: https://girlinaudiophilia.wordpress.com/2016/05/16/thoughts-on-prodj100/
Introduction
The Koss ProDJ100 is a pair of portable, closed, over-ear headphones currently priced at ~$50 on Amazon. They are known (or not) for their good build...

I've tried about two dozen pairs of headphones and I'm always looking for the absolute best pair for vocals. I also want forward sounding mids, but not too bad. My preference is also for highs that are not harsh or fatiguing. Being somewhat rolled off is ok. I find the SRH-840 is what I like in this area.
The best thing about this headphone is the quality of vocals. It just sounds amazing with both male and female vocals. This headphone is absolutely perfect for Jpop, Cantopop, Imogen Heap and Radiohead. The strange thing about this headphone is that the sound feels like an IEM. You feel like you're right there.
The highs are somewhat rolled off, but not as badly as the...

Like tdockweiler, I'm a fan of forward mids and these definitely bring it. They are excellent for vocals in general, not just for male vocals. They're relatively neutral sounding with a tight, controlled bass that doesn't extend too low. They're far from being "extreme bass" DJ headphones and I wouldn't suggest them to someone who is a fan of real bass heavy electronic music -- I hardly noticed the subbass on these headphones, even when equalizing it up. It is present, but not thumping like I would want for that type of music. One issue I noticed was that if you didn't take the time to get a proper seal, it completely ruined the bass. The DJ 100s were very dependent on being placed...

I have about 12 hours into this set of phones and I am impressed they do just about everything well not great. I have used them on my PS3 playing BFBC2 and I can hear things I have never heard before. I am able to locate where bullets are coming from and when someone is coming up the stairs. They are not the best for gaming but they will do the trick if you are on a budget and want something well balanced in all areas. I have tried Rock, Metal, and Rap and the DJ100 seem to take them all really well. I have watched a few Blu-Rays though them too and they seem to do a good job wish there was just a little more bass in them for the explosions and when tanks drive by same when I was...

Pros: Really brings out the vocals without endangering lows/highs, very well balanced, amazing clarity

Cons: Not very comfortable but that can be remedied with different pads.

This is a great inexpensive headphone with one of my favorite sonic signatures. It's one of the few headphones I bought twice when I tried much more expensive headphones being disappointed and realizing what I had lost.
One of the things I look for in headphones is how vocals are represented and these probably beat out most 200 dollar headphones in vocals department. But these are not one focus headphones. They're for studio use so they are well balanced everything is tight and well represented. If you're looking for the humming type of bass found in Beats headphones, you won't get them here. If you're looking for crazy sibilant highs that crackle, they were smart about that too,...

Pros: Great Sound Quality, Very Well Built, very good value for under $100

Cons: Not especially comfortable, coiled cable is a little heavy for walking around.

I got mine for $35, and I can't figure out why. They sound perfect to me, with forward mids, clear vocals, and bass that thumps, but isn't in your face. Simply put: if you find these for under $80, snatch em up!

Bass: Definitely not an "extreme bass" headphone like the box says. There is a very slight bass tilt. Very tight bass. Very well extended with m50 pads.
Mids: Pretty forward overall. Amazing with vocals. Amazingly smooth mids.
Highs: Has a little bit of treble roll off. Sounds a little thin
The overall clarity and detail is great.
These really work better with an amp. Overall clarity, soundstage, and neutrality improves.
Effects of Ath m50 pad:
Comfort
Bass extension
Tames mids a little (still has forward mids)
Adds a little more depth and a lot of width to soundstage (still nowhere near open headphones of course )

So, after many many hours of research I finally found a headphone that sounds good and it doesn't go over my limit of $70 in Canada. If I had lived in the U.S. I maybe would have picked up the CAL! instead... but I'm sure glad I didn't! Some impressions:
Comfort:
Here's where they fall really short. Comfort. My ears are rather large so the koss sitting on my ears for hours on end (I usually get home, put on my headphones and don't take them off unless I go eat/restroom or go to sleep) gave me some earaches. It wasn't horrible but was uncomfortable nonetheless. However, I've heard that m50 pads enhance them so I decided not to do a review until I get them. Until then I taped...

Pros: Easy to drive, stylish for full sized heaphones, nice combo of flavor and accuracy

Cons: Sucked out lower midrange

Simply put? The lower midrange just isn't there.
Everything else, though, is pretty damned nice. Vocals? Clear as hell. Bass? Plenty of impact, and it gets LOW*
With that out of the way though, the lower midrange has problems. If I was feeling mean I'd some up with something witty, but **** you.
You like powerful vocals, orchestras, or metal? You better start running and screaming. Like R&B? Stick around, these will do you well.
*modded with m50 earpads, at least. Without them the midrange changes for the better, but they're otherwise unremarkable, and quite a bit less comfortable.

I have recently been on an all-consuming quest to find a new set of headphone due to my renewed interest in audiophilia. Anyways, for the past while I have been intently auditioning several pairs of headphones to replace my aged AKG K240DFs (bought in the very late 80s, but just lack the “POW” I want).
Just to let you know, my requirements are for a closed design headphone because I often listen music in bed and don’t want to disturb the better half (happy wife, happy life – you married guys know it).
So based on internet reviews (and posts on this forum from tdockweiler who is the PRODJ100’s top evangelist!) I bought the Koss PRODJ100 headphones and was really...